The field of electronic vehicle tracking for tolling and other purposes has seen many iterations over the years. These include the use of vehicle-based backscatter transponders that are detected by and communicate with by roadside equipment, active transponders that are detected by and communicate with by roadside equipment, hybrid transponders having both active and backscatter functions, and video monitoring of vehicle license plate and other placards. Cellular telephones have also been described for use in tolling systems, alone or in combination with the aforementioned types of transponders. These transponders have been implemented in various ways and using combinations of software running on embedded microcontrollers, Application Specific Integrated Circuits (ASICs) (both digital and mixed signal types), discrete logic, digital, and analog circuitry. The transponders may operate in a passive manner whereby a downlink signal (reader to tag) is sent to the tag which includes modulating data on an ultra-high frequency (UHF) carrier signal to the tag including commands to perform certain operations. These tags are powered by harvesting incoming energy from a radio signal such as the UHF carrier signal in the 900 MHz range, which powers the transponder circuitry. The tag typically backscatter modulates the incoming continuous wave (CW) signal to create an uplink (tag to reader) signal in accordance with the said received commands. These types of tags are currently used in the Florida SunPass® system, for example. Tag protocol also may also be used in conjunction with an active transmitter for both downlink and uplink, such as is used in the EZ-Pass® system.
Radio Frequency Identification transponders (RFID tags) used for vehicle tolling are typically programmed at the factory with a unique user identification number that is used to identify the owner of the tag. In the United States, many different tolling protocols are used in the various toll agencies and often tags from one agency are not compatible with tags in another. This incompatibility requires users who would like to utilize toll roads across multiple toll agencies to use multiple toll tags in their vehicle. Multi-protocol tags can work with the various toll agencies and their protocols, however, these tags are programmed at the factory, at time of manufacture and communicate only with dedicated road-side equipment.